The Siemens SENTRON 5SY4520-6 is a 2-pole (1P+N) miniature circuit breaker rated 20 A at 35 °C with a B tripping characteristic. It's not for motor starting; that's a C or D curve.
Breaking capacity and standards
That dual rating is standard for MCBs that carry both the household (EN 60898) and industrial (IEC 60947-2) marks. The 10 kA figure governs typical domestic / commercial fault levels; the 20 kA figure applies when the MCB is used as a circuit breaker under the industrial standard, which allows higher prospective fault currents. Also carries UL1077 recognition as a supplementary protector — useful for North American panels where the MCB is not the main service disconnect but backs up branch protection.
Thermal derating — the real-world current
The 20 A rating holds at 35 °C ambient. Above that, the thermal-magnetic trip unit derates: 18.78 A at 40 °C, 18.14 A at 45 °C, 17.48 A at 50 °C, 16.8 A at 55 °C, 16.08 A at 60 °C. If your panel runs warm — say 50 °C inside a sealed enclosure — you're effectively getting a 17.5 A breaker. Plan the load accordingly; don't size to the 20 A nameplate if the ambient is above 35 °C.
Installation depth is 70 mm; the overall depth is 76 mm, so it clears a standard 80 mm deep enclosure.